Interconnection of electronic devices has become a significant factor in determining the performance of electronic systems in recent years. This is due in part to the trend toward higher device integration, i.e., large scale integrated circuits with several million active transistor elements, sub-micron feature sizes and I/O pinouts approaching 400-500 connections, along with the associated increase in silicon real estate size, typically greater than 0.50 inches, and power dissipations of greater than five watts per IC. These factors certainly pose major technology problems for the electronics packaging engineer, especially in the area of single chip packaging. Present IC packaging design approaches, material systems, and technologies for single chip packaging have become, in the high-end integrated circuits, significant performance differentiators and, as such, comprise major product differentiators.
The emerging need to integrate off-chip interconnect with tailored electrical characteristics of controlled impedance transmission lines, low-loss power and ground distribution networks and minimal signal paths has driven the present leading edge subsystem components to consider a form of electronics packaging known within the industry as Multi-Chip Modules, or MCM. An MCM typically constitutes the ability to package multiple bare ICs in a single subsystem package, where chip-to-chip interconnect is supported by one of a number of high performance interconnect approaches internal to the next system packaging level interface. There are at present three major classes of MCM packaging approaches.
The first class of MCM has come to be known as MCM-L, where the "L" stands for Laminate technology. This approach typically consists of an extension of standard printed wiring board technology that supports fine dimension surface features to enable 6-10 mil lead pitch interfaces for a perimeter pad device to provide for the interconnect. Electrical connections for a plurality of ICs are generally made with a Tape Automated Bonding (TAB) lead interface from the outer perimeter bond pads on the device to a fine lead pitch perimeter pad interfaced on the Laminate technology interconnect. When multiple bare ICs having TAB lead frames attached to the I/O bond pads are interconnected with this approach, an MCM system is created; hence, the term MCM-L. Alternate methods of device-to-Laminate interconnect electrical connection can be used. For example, bare chip wire bond connections and either solder bump or electrically conductive adhesive bonds are in use or in development within the industry. This technology has become accepted as a low-cost manufacturing approach for the low-to-mid range performance (typically 20-70 MHz clock frequency) products and has the significant advantages of availability, low-cost, low-risk and the ability to leverage existing packaging technology into the emerging high-volume MCM packaging applications.
The second class of MCM packaging technology is known as MCM-C. This approach is an extension of both the hybrid practice, where "thick film" metallizations can be screen printed and then fired in multiple layers on a ceramic substrate, and the multi-layered co-fired single chip packaging technologies, where screened metallizations are printed on ceramic in the "green tape" or pre-fired state and the ceramic tape and metallization are cured in a single process. In either case, an interconnect of several layers is formed on a ceramic substrate, which can also serve as a package to interconnect to the next system level, and a plurality of ICs can then be connected to this wiring structure by a variety of techniques. Among the methods for IC connection that are utilized are: wire bond, flip chip solder bump, TAB, thermal compression bond, TAB solder interface and electrically conductive polymer adhesive. This technology has the advantage of significant leverage of an installed base of ceramic interconnect manufacturing due to its reliance on extensions of single chip packaging technologies and hybrid interconnect technologies. However, there are penalties in interconnect wiring densities due to the limitations in feature sizes that can be achieved, and in general the electrical performance characteristics of the ceramic wiring substrate, which present limitations not suitable for leading technology IC interconnect.
The third class of MCM packaging technology is known as MCM-D, where "D" stands for Deposited (metallization). This approach leverages thin film process techniques that are typically extensions of IC manufacturing processes. Because of this, interconnect feature sizes much finer than the present art MCM-L or MCM-C features are achievable. MCM-D interconnects are typically constructed of dielectric layers of polyimide based materials with either aluminum or copper and barrier metal conductors in multiple layer structures. These thin film multi-layer MCM-D wiring interconnects are built using a variety of materials for mechanical substrates, which provide a manufacturing tooling plate for processing, and mechanical and thermal structures for next level package interface. Substrate materials that can be used include silicon, ceramics, glass and metal matrix composite materials. MCM-D thin film interconnects generally display the best attributes of electrical performance characteristics in the smallest packaging size and thermal penalty attributes. However, they also exhibit the highest manufacturing costs at present and due to this, are limited for consideration to only the interconnect applications that demand the highest electrical and mechanical packaging solutions.